The present invention relates to pulse oximeter sensors, and in particular to methods and apparatus for preventing the shunting of light between the emitter and detector without passing through blood-perfused tissue.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light scattered through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light scattered through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have typically been provided with a light source that is adapted to generate light of at least two different wavelengths, and with photodetectors sensitive to both of those wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, an ear or the scalp. In animals and humans, the tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the sensor.
One problem with such sensors is the detection of ambient light by the photodetector, which can distort the signal. Another problem is the shunting of light directly from the photo-emitter to the photodetector without passing through blood-perfused tissue. FIG. 1 illustrates two different types of light shunting that can interfere with proper detection of oxygen saturation levels. As shown in FIG. 1, a sensor 10 is wrapped around the tip of a finger 12. The sensor includes a light emitter 14 and a light detector 16. Preferably, light from emitter 14 passes through finger 12 to be detected at detector 16, except for amounts absorbed by the blood-perfused tissue.
A first type of shunting, referred to as type 1 shunting, is shunting inside the sensor body as illustrated by light path 18, shown as a wavy line in FIG. 1. Light shunts through the sensor body with the sensor body acting like a light guide or light pipe, directing light from the emitter to the detector.
A second type of shunting, referred to as type 2 shunting, is illustrated by line 20 in FIG. 1. This type of light exits the sensor itself, but reaches the detector without passing through the finger. In the embodiment shown, the light can go around the side of the finger, perhaps by being piped by the sensor body to the edges of the sensor and then jumping through the air gap between the two edges which are wrapped around the side of the finger.
The problem of light shunting can be exacerbated by layers placed over the emitter and detector. Often, it is desirable not to have the emitter and detector in direct contact with the patient's skin because motion artifacts can be reduced by placing a thin layer of adhesive between these components and the skin. Thus, the emitter and detector are typically covered with a clear layer which isolates them from the patient, but allows light to transmit through. The feature of allowing light to transmit through the layer also provides the capability for the clear layer to provide a wave guide effect to shunt light around the finger to the detector.
Such layers covering the emitter and detector can be originally included in the sensor, or can be added during a reinforcing or modifying procedure, or during a remanufacture of the sensor. In a remanufacture of a sensor, a sensor which has been used may have its outer, adhesive transparent layer removed. Such a layer is shown in FIG. 2 as a transparent layer 22 over a sensor 10. Layer 22 is an adhesive, transparent layer placed over a substrate layer 24, upon which emitter 14 and detector 16 are mounted, along with any other associated electronics. Layer 22 thus serves both to protect the emitter and detector from the patient, and to adhere the sensor to the patient. During remanufacture, this layer can be stripped off, and a new layer placed thereon.
Alternately, layer 22 may be left in place. Such a sensor, with an adhesive outer layer, may be a disposable sensor, since it would not be desirable to have the same adhesive used from one patient to another, and an adhesive is difficult to clean without removing the adhesive. Accordingly, a modification of such a sensor may involve laminating sensor 10 to cover over the adhesive, by adding an additional lamination layer 23 (shown partially broken away) over layer 22. The lamination layer is itself another layer for shunting light undesirably from the emitter to the detector. Once laminated, in one method, the sensor is then placed into a pocket 26 of a sheath 32. Sheath 32 includes a transparent cover 28 on an adhesive layer 30. Layer 30 is adhesive for attaching to a patient. Layer 28 may also optionally be adhesive-coated on the side which faces the patient. Such a modified sensor can be reused by using a new sheath 32. Transparent layer 28 forms yet another shunting path for the light.
A commercially available remanufactured sensor, similar in design to the sensor of FIG. 2, is available from Medical Taping Systems, Inc. Another example of a sheath or sleeve for a sensor is shown in U.S. Pat. No. 4,090,410, assigned to Datascope Investment Corp.
In addition, when a sheath such as 32 is folded over the end of a patient's finger, it has a tendency to form wrinkles, with small air gaps in-between the wrinkled portions. The air gaps can actually exacerbate the shunting problem, with light jumping more easily through the air gaps from one portion of the transparent layer to another.
Other types of sensors have not used a solid transparent layer 22 as shown in FIG. 2. For instance, the Nellcor Puritan Bennett R-15 Oxisensor.RTM. and N-25 Neonatal/Adult Oxisensor products use a white-colored substrate with separate transparent strips placed over the emitter and detector (such as strips 11 and 13 illustrated in FIG. 1). The transparent strips are adhesive for adhering to the patient. Since two strips are used, an air gap (gap 15 in FIG. 1) occurs between the transparent layers. As noted above, light can jump such an air gap, and thus a gap by itself may not eliminate all shunting problems. The use of a dark-colored substrate may reduce the amount of shunting, if the selected color is opaque to the wavelengths of interest from the emitter, 650 nm red and 905 nm infrared in a typical implementation. However, the white substrate typically used in the R-15 and N-25 sensors is substantially translucent and thus has limited light blocking qualities.
It has been found that shunted light can significantly affect the accuracy of oxygen saturation readings using a pulse oximeter. Accordingly, there is a need to develop a barrier to such light to improve the accuracy of pulse oximeter sensors.